Date: Sun, 25 Nov 2001 01:42:00 +0100 From: Maxime Henrion <mux@qualys.com> To: Hiten Pandya <hitmaster2k@yahoo.com> Cc: current@FreeBSD.org Subject: Re: some info Message-ID: <20011125014200.A831@nebula.noos.fr> In-Reply-To: <20011125002202.19785.qmail@web21109.mail.yahoo.com> References: <20011125002202.19785.qmail@web21109.mail.yahoo.com>
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Hiten Pandya wrote: > hi all, > sorry for running -current.. but i am an enthusiastic > and challenging man...(boy)... > > anyways.. whats a mutex and a lock order reversal... > if you could point to some good manual on these > subjects... thanks... > > help is appreciated... > > thanks again... A mutex is an algorithmic object used to serialize operations. It stands for ``mutual exclusion''. It's useful when the same code is ran several times in parallel. For example, if two threads wanted to modify a linked list in the same time, there is a chance that the linked list would get corrupted since an insert or remove operation is not atomic (one of the thread could get preempted when it has not finished to remove or insert an element, and thus the linked list is not in a normal state). In such cases, every part of the code that modify the linked list has to obtain the mutex before doing it and release after. If another thread tries to acquire the mutex, it will block until the first thread has released it. When some code has to obtain two locks or more, deadlocks might happen. If thread 1 has lock A and tries to acquire lock B while thread 2 has lock B and wants lock A, then both threads will block indefinitely. To solve this, one way is to always obtain the lock in the same order. The warning messages you got show that some code is violating this lock order somewhere. I found ``Unix Internals'' from Uresh Vahalia to be a very good book on this topic. Hope this helps, Maxime Henrion -- Don't be fooled by cheap finnish imitations ; BSD is the One True Code To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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